In-plane switching type (hereafter referred to as “IPS type”) image display apparatus have been proposed in the field of image display apparatus that display images using, for example, the electro-optical effect of liquid crystal molecules. The IPS type of image display apparatus applies an electric field to direct the orientation of the liquid crystal molecules in a direction parallel to the surface of the substrate that holds the liquid crystal layer containing liquid crystal molecules. Since the IPS type image display apparatus have superior characteristics in terms of the voltage retaining properties and the view angle compared with conventional image display apparatus, they have been considered to be particularly promising recently.
FIG. 9 shows the basic configuration of an array substrate that constitutes a conventional IPS type image display apparatus. As shown in FIG. 9, the array substrate of the IPS type image display apparatus has a structure in which common electrodes 102 and 103 are deployed away from pixel electrode 101 in the horizontal direction, and auxiliary electrode 104 is deployed under pixel electrode 101. A thin film transistor 105 that functions as a switching element is deployed in the vicinity of a pixel electrode 101. One source/drain electrode of thin film transistor 105 is connected to the pixel electrode 101, and the other source/drain electrode is connected to signal line 107. Furthermore, the gate electrode of thin film transistor 105 is connected to scanning line 106. Common electrodes 102 and 103 are connected to constant voltage supplying circuit 108 that supplies a constant electric potential; auxiliary electrode 104 is also connected to constant voltage supplying circuit 108, and each is maintained at a constant electric potential.
Scanning line 106 is given a prescribed electric potential to drive thin film transistor 105, and the electric charge supplied from signal line 107 is accumulated in pixel electrode 101. Since common electrodes 102 and 103 maintain a constant electric potential, an electric potential difference corresponding to the accumulated electric charge arises between pixel electrode 101 and common electrodes 102 and 103, and an electric field arises in a direction parallel to the array substrate. An IPS type image display apparatus has a structure in which a large number of pairs of pixel electrode and common electrodes shown in FIG. 9 are deployed corresponding to the number of display pixels, and a prescribed number of signal lines and scanning lines are deployed corresponding to such pairs. This apparatus performs the aforementioned operation to each pixel electrode, and displays images by using the electro-optical effect that arises in the liquid crystal layer encapsulated on the array substrate.
Auxiliary electrode 104 is deployed under pixel electrode 101, and is deployed in such a way that the auxiliary electrode 104 overlaps with pixel electrode 101 via a prescribed dielectric layer. As a result, the pixel electrode 101, the dielectric layer, and auxiliary electrode 104 form an auxiliary capacitance. This auxiliary capacitance has the role of stabilizing the electric potential of pixel electrode 101. The scanning line 106, signal line 107 and such that are deployed near pixel electrode 101 are designed to have electric potentials that vary over a prescribed range, and the auxiliary capacitance is provided to prevent this electric potential variation from influencing pixel electrode 101. In view of the stability of the electric potential of pixel electrode 101, it is preferable that auxiliary electrode 104 maintain a constant electric potential. Therefore, auxiliary electrode 104 is connected to constant voltage supplying circuit 108, and as a result, auxiliary electrode 104 and common electrodes 102 and 103 are maintained at an identical electric potential.
In common image display apparatus including IPS type image display apparatus, signal lines, scanning lines, and prescribed electrodes are insulated from each other, which results in a tendency to hold electrostatic charge during manufacturing. For example, during a film forming process in a clean room, friction between the surface of the array substrate and the source gas flowing in the clean room can cause electrostatic charging. Thus, for example, if signal line 106, which is connected to the gate electrode of thin film transistor 105, is electrostatically charged, the gate potential of thin film transistor 105 might exceed an allowable limit. If the gate electric potential exceeds an allowable limit causing destruction of the thin film transistor, the thin film transistor 105 can no longer supply the electric charge to a corresponding pixel electrode, which prevents proper operation of the image display.
For a mechanism to discharge the electrostatic charge to the outside, shunt line 109 and switching element 110 are deployed on the array substrate, as shown in FIG. 9. Specifically, shunt line 109 is electrically connected via switching element 110 to scanning line 106, and shunt line 109 is also connected to constant electric potential source 108.
Switching element 110 is formed by an active element that is a combination of a plurality of thin film transistors, for example. Switching element 110 is designed to turn on and conduct a current between shunt line 109 and scanning line 106 when the electric potential difference between shunt line 109 and scanning line 106 reaches or exceeds a prescribed value. Switching element 110 is formed in such a way that the electric potential of scanning line 106 at the time when switching element 110 turns on is lower than the electric potential at which thin film transistor 105 is destroyed.
Electrostatic charge in scanning line 106 can be discharged to the outside by connecting scanning line 106 and constant voltage supplying circuit 108 via switching element 110 and shunt line 109. Specifically, when scanning line 106 is electrostatically charged and the electric potential of scanning line 106 reaches or exceeds the prescribed value, switching element 110 is turned on based on the electric potential difference between scanning line 106 and shunt line 109, and now there is electrical conduction between scanning line 106 and shunt line 109. Therefore, the electrostatic charge in scanning line 106 flows out via switching element 110 and shunt line 109 to constant voltage supplying circuit 108, and the electric potential of scanning line 106 decreases down to a level equivalent to the level before the electrostatic.
However, a conventional IPS type image display apparatus as shown in FIG. 9 has various issues. First, the image display apparatus shown in FIG. 9 may experience burn-in, which occurs when a constant image is displayed on the screen for a long period of time before switching to a different image, resulting in the previous image faintly remaining on the screen.
The burn-in phenomenon occurs particularly in the area corresponding to an end portion of auxiliary electrode 104. Despite attempts to eliminate the burn-in phenomenon, no effective countermeasure has been provided.
Another issue associated with the image display apparatus of FIG. 9 is that, during array substrate testing, units that can be turned into non-defective units are erroneously recognized as defective units. As shown in FIG. 9, scanning line 106 and connecting line 111 are deployed in such a way that they partially overlap, with an insulation layer provided in between to prevent electrical conduction between scanning line 106 and connecting line 111. However, if electrical conduction occurs between scanning line 106 and connecting line 111 for some reason (such as due to a short circuit defect), the drive state of thin film transistor 105 cannot be controlled any more and a line-like display defect and/or dot-like display defect or color heterogeneity occurs, resulting in deteriorated image quality.
Because of this, after completion of the manufacturing process of the image display apparatus, usually a prescribed electric potential is applied on scanning line 106 to test if any current flows into constant voltage supplying circuit 108. The presence of an inflow current is deemed to indicate the presence of electrical conduction between scanning line 106 and connecting line 111 and the image display apparatus subjected to the test is diagnosed as defective.
In the case of the image display apparatus of FIG. 9, there are cases when the inflow current exists in the aforementioned test but there is no electrical conduction between scanning line 106 and connecting line 111. As shown in FIG. 9, shunt line 109 and scanning line 106 partially overlap with each other and electrical conduction sometimes occurs between shunt line 109 and scanning line 106 just as in the case of connecting line 111. In this case, a current also flows into constant voltage supplying circuit 108 if a prescribed electric potential is applied on scanning line 106.
As described above, shunt line 109 is provided to prevent thin film transistor 105 from being destroyed by electrostatic charging at the time of manufacturing of the array substrate. Therefore, once the image display apparatus is completed shunt line 109 does not have a particular function. Even when there is electrical conduction between shunt line 109 and scanning line 106, there is no particular problem in using the image display apparatus if shunt line 109 and constant voltage supplying circuit 108 are disconnected from each other.
However, in the case of the image display apparatus shown in FIG. 9, there is no way to tell whether the electrical conduction is occurring between the scanning line 106 and the connecting line 111 or shunt line 109. Therefore, when a current flows into constant voltage supplying circuit 108 in the test, there is no other choice than to discard the unit as defective, which results in lower manufacturing yield.